1. Field of the Invention
The present invention relates to a data storage apparatus as a hard disk drive. More particularly, the present invention relates to a method for deciding whether or not an error has occurred in each data read from a data storage medium and further a method employed for error data correction.
2. Background Art
A hard disk drive is one of the most popular external storage apparatuses employed for computers. As known well, a magnetic disk, which is a storage medium of such a hard disk drive, consists of sectors (the minimum unit of recording) formed by radially partitioning tracks on the surface of the disk. The tracks are formed by dividing the surface of the disk just like the growth rings. The line recording density of such the magnetic disk can be increased as closing to the outer periphery. This is why the “zoned bit recording” method, which is the present mainstream of data recording methods of magnetic disks, divides every track into some zones and fixes the line recording density in each of those zones. Consequently, the closer the track comes to the periphery of the magnetic disk, the more the number of sectors in the track increases, since the sector length is usually fixed at 512 bytes.
A hard disk drive is provided with a head used to read/write data from/on its magnetic disk. This head is attached to an actuator mechanism driven by a VCM (Voice Coil Motor).
When the head is to read/write data from/on the magnetic disk, the actuator mechanism is driven, thereby the head moves to a predetermined track so as to be positioned there. The head movement to a predetermined position is controlled according to the servo information stored on the magnetic disk.
In such a hard disk drive, the recording density per unit area increases significantly then the signal processing system S/N ratio lowers, thereby the existence of magnetic defects on the magnetic disk comes not to be negligible. This is why error correction becomes a very important factor to improve the reliability of data in that the data can be read/written even when a magnetic disk has such defects.
Error correction is executed, for example, for read data that contains an error. The read data errors are usually caused by peeling of the magnetic layer from the subject magnetic disk, scratches on the magnetic disk formed by rubbing against the magnetic head, dust and foreign matters stuck on the magnetic disk, etc. A hard disk drive enables a few tens of bytes of parity data to be added to each sector (usually, 512 bytes) of data on the magnetic disk so as to be used for such the error correction. This parity data is referred to as the ECC (Error Correcting Code). When reading, a syndrome is generated from the data read by the ECC circuit located in the subject hard disk drive and the read parity (ECC). This syndrome is used to detect each error location and correct the error.
For example, the four-way interleaving is taken here as an example of such the error correction for data divided in units of 512 bytes/sector. The four-way interleaving means a method that divides a sector of data into four sections (interleave sections) sequentially from start to end and finds the parity in each interleaved data. In this case, when the addition of ECC byte is performed with 12 bytes per interleave section, that is, 48 bytes per sector, 5-byte errors per interleave section, that is, 20-byte errors per sector can be corrected. Consequently, when the number of error-occurrence bytes in reproduced data is within the predetermined capacity, error correction can be continued without stopping the data transfer while the data is read sequentially. This operation is referred to as OTF (On The Fly Collection).
When the number of error-occurred bytes exceeds the error correcting capacity, however, the MPU and/or the HDC (Hard Disk Controller) of the hard disk drive retries reading by changing the read parameters such as the offset in the track width direction of the read head, the coefficient of the channel equalizer, etc. This is referred to as an error recovery operation. And, when an error cannot be corrected even with this error recovery operation, if the error location is identified by any means, more error bytes can be corrected than in the error correction by the OTF. This operation is referred to as “erasure correction.” Conventionally, each error location has been detected with use of error location information obtained according to a parity error signal, etc. received from a read/write channel Viterbi Decoder as disclosed, for example, in Published Unexamined Patent Application No. 2000-57707.
However, the above conventional method has often been confronted with problems. For example, error locations are found excessively due to the variation of such characteristics as the soft error rate, etc. among respective heads. Especially, when random noise is a factor of an uncorrectable error, the error location might be varied each time the data is read therefrom. In addition, the accuracy of the viterbi decoder to find error locations has often become unstable. This has also been a problem. This is why the error correction probability has not been so high when the error correction capacity is exceeded as described above.